Preforms, in particular plastic preforms, are widely used for producing a variety of products ranging from curved surfaces to beverage bottles, for instance. Commonly, before forming such preforms into a desired shape, they have to be heated up to a temperature close to the melting point of the material of the preform. Shaping tools will then alter the shape so that a completely new product evolves. Such shaping methods include deep-drawing or blow-moulding of plastic preforms.
A well-known application of (blow-)moulding heated preforms is the production of PET (polyethylene therephthalate) bottles which are used for a variety of beverages such as mineral water, juices, lemonade and beer. In order to produce such bottles, a PET preform having a tubular shape is heated by means of halogen lamps. FIG. 1 shows a schematic sectional view of such a heating arrangement according to the state of the art. A body 1 of a preform having a first, outer, surface 2 and a second, inner surface 4 is heated by three halogen lamps 5. For that purpose, mirrors 7 are used to reflect parts of the divergent light 3 emitted by the halogen lamps 5 and to direct the light rays essentially into a traversal direction T of the preform 1. The traversal direction T is defined by a shortest direct line between the first surface 2 and the second surface 4 at a point where the light is coupled into the body 1. Because the light 3 is divergent, i.e. undirected or only partially directed, and consisting of light of many different wavelengths, it does not completely traverse the body 1 in the traversal direction T, but this direction is nevertheless the principal direction of traversal in general. Usually, preforms are moved on a production line along which a multitude of halogen lamps 5 are arranged. This leads to an increase of the temperature to a point where the preforms can be shaped by blowing them inside a mould or also by merely pressing them into the mould.
Halogen lamps emit a broad spectrum of visible and invisible light rays which ranges into the infrared region, as can be seen in FIG. 2. Here, the wavelength spectrum of a halogen lamp (in nanometers) is plotted on the x-axis, while the left y-axis refers to the corresponding absorption spectrum of PET in % and the right y-axis refers to the emission energies of typical halogen lamps in Watts. The first curve A corresponds to the PET absorption spectrum (left y-axis) while the second curve B corresponds to the emmission energy spectrum (right y-axis). It can be observed that the absorption of PET is notably low in the visible and near infrared wavelength ranges up to about 1010 nm, while a higher absorption rate of PET can be realized in between 1010 nm and 2000 nm. Above 2000 nm, PET is basically opaque. The wavelength spectrum of halogen lamps therefore produces an in homogenous heating result, since a significant portion of the halogen lamp spectrum is at wavelengths with a very high absorption by PET, i.e. above 2000 nm. Therefore, the larger portion of the emitted light is absorbed at the outer part of the preform, while its inner part is heated to a much lower extent. For that reason, it is often necessary to cool down the outside of a preform, for instance by spraying water on it, while prolonging the heating process at the same time in order to get the inside of the preform heated up as well. In sum, this leads to a more energy-consuming and longer heating process than would be necessary if a homogeneous heating was applied.
A way to circumvent these drawbacks is to choose a different light emitting system, such as lasers, which only operates at a certain wavelength. This way, the wavelength can be adjusted to the necessities of the heating process, which are mainly determined by the material and the thickness of the preform. For instance, heating a PET preform by means of laser wavelengths at an absorption rate by PET of less than 50% would mean that a more continuous absorption could be achieved, resulting in a lower overall energy input being necessary, i.e. the heating process could be carried out in a more effective way. However, such suitable laser wavelengths are not emitted by typical lasers for everyday use which emit at typical wavelengths of 800 or 970 nm. Unfortunately, in this wavelength range, only a quite low absorption rate of PET of about 15% can be achieved.
Against this background, it is highly desirable to provide a possibility to heat a body of a preform by means of a laser beam—or more broadly—by means of a directed light beam, more effectively and with less regard to the wavelength of the light beam which a light source emits.